Implement fast skip of "keep all" pruning

Instead of configuration like this:

``` yaml
pruning:
  keep:
    - type: "regex"
      regex: ".*"
```

or like this:

``` yaml
pruning:
  keep_sender:
    - type: "regex"
      regex: ".*"
  keep_receiver:
```

which keeps all snapshots, now it's possible to omit `pruning:` at all, or one
of `keep_sender:` or `keep_receiver:`. In this case zrepl will early abort
pruning and mark it as done.

Originally zrepl requests all snapshots and does nothing after that, because
pruning configured to keep all snapshots, but anyway it spends some time
executing zfs commands.

README.md

@@ -156,6 +156,35 @@ This project is a fork of [zrepl](https://github.com/zrepl/zrepl).
     type: "cron" still works too, just for compatibility. Both of them is the
     same type.
 
+  * Fast skip "keep all" pruning.
+
+    Instead of configuration like this:
+
+    ``` yaml
+    pruning:
+      keep:
+        - type: "regex"
+          regex: ".*"
+    ```
+
+    or like this:
+
+    ``` yaml
+    pruning:
+      keep_sender:
+        - type: "regex"
+          regex: ".*"
+      keep_receiver:
+    ```
+
+    which keeps all snapshots, now it's possible to omit `pruning:` at all, or
+    one of `keep_sender:` or `keep_receiver:`. In this case zrepl will early
+    abort pruning and mark it as done.
+
+    Originally zrepl requests all snapshots and does nothing after that, because
+    pruning configured to keep all snapshots, but anyway it spends some time
+    executing zfs commands.
+
   * Small cosmetic changes
 
 ## User Documentation

config/config.go

@@ -103,7 +103,7 @@ type PassiveJob struct {
 type SnapJob struct {
 	Type             string            `yaml:"type"`
 	Name             string            `yaml:"name"`
-	Pruning          PruningLocal      `yaml:"pruning"`
+	Pruning          PruningLocal      `yaml:"pruning,optional"`
 	Snapshotting     SnapshottingEnum  `yaml:"snapshotting"`
 	Filesystems      FilesystemsFilter `yaml:"filesystems"`
 	MonitorSnapshots MonitorSnapshots  `yaml:"monitor,optional"`
@@ -266,8 +266,8 @@ type SnapshottingManual struct {
 }
 
 type PruningSenderReceiver struct {
-	KeepSender   []PruningEnum `yaml:"keep_sender"`
-	KeepReceiver []PruningEnum `yaml:"keep_receiver"`
+	KeepSender   []PruningEnum `yaml:"keep_sender,optional"`
+	KeepReceiver []PruningEnum `yaml:"keep_receiver,optional"`
 }
 
 type PruningLocal struct {

daemon/pruner/pruner.go

@@ -212,6 +212,12 @@ func (s State) IsTerminal() bool {
 type updater func(func(*Pruner))
 
 func (p *Pruner) Prune() {
+	if len(p.args.rules) == 0 {
+		l := GetLogger(p.args.ctx)
+		l.Info("skip pruning, because no keep rules configured")
+		p.state = Done
+		return
+	}
 	p.prune(p.args)
 }